[69Cobra]

You should have your main starter cable that runs from the solenoid down to the starter. That goes to the main post for the starter motor but then you should also have another smaller terminal for the starter solenoid that's on the starter itself. Just take a #10 size wire with a ringed terminal end on it and jumper between the two post. Now when you hit the key and it pulls in the factory solenoid on the inner fender well it will also energize the solenoid at the starter and pull the starter in to spin the motor.
Hope that helps.